A review and meta-analysis of the effects of climate change on Holarctic mountain and upland bird populations

Mountain regions are globally important areas for biodiversity, but are subject to multiple human-induced threats, including climate change, which has been more severe at higher altitudes. We reviewed evidence for impacts of climate change on Holarctic mountain bird populations in terms of physiology, phenology, trophic interactions, demography, and observed and projected distribution shifts, including effects of other factors that interact with climate change. Studies were focussed on an objective classification of Holarctic mountain birds, defined as those species for which the majority of their breeding range occurred in mountain regions. Our review showed that mountain bird populations have already responded to climatic (extreme weather events, temperature, rainfall and snow) and environmental (i.e. land use) change, but we know little about the underlying mechanisms, nor about the synergistic effects between climate and land use and their relative contribution to changes in avifauna. Few studies have considered shifts in elevational distribution over time and our meta-analysis did not find a consistent direction in elevational change. However, shifts towards higher elevations were more positive when rates of temperature change were higher. A meta-analysis carried out on future projections of distribution shifts suggested that mountain birds are likely to be more negatively impacted than other non-mountain species. Birds inhabiting high altitude areas are expected to be greatly affected directly by changes in climate, while those living in ecotone and forest systems will most likely be influenced by a combination of changes in land use and climate. Adaptation responses to climate change mostly rely on managing and extending current protected areas for both native species, and for expected immigrating species losing habitat and climatic suitability at lower elevation. However developing effective management actions requires an improvement in the current knowledge of mountain species ecology, in the quality of climatic data, and in understanding the role of interacting factors. Scientists should provide valuable tools to assess the status of mountain birds (i.e. through the development of a mountain bird index), while policymakers should influence legislation to develop efficient agri-environment schemes and forestry practices for mountain birds, as well as to regulate leisure activities at higher altitudes.